Our studies on the structure of fibrinogen are primarily aimed at finding how this molecule is transformed into fibrin. To this end we are attempting to determine the entire primary structure of this molecule as it exists in three species: human, bovine and lamprey. Our reasoning is that the most critical features of the molecule ought to be conserved during evolution. We are also combining our primary structure studies with certain physico-chemical approaches, including electronic birefringence and electron microscopy. It is our hope to obtain the approximate relative locations in space of various key structural components of the fibrinogen molecule.